1. Field of the Invention
The present invention relates to a current-resonant power conversion apparatus employing a switching system and a method of controlling such a power conversion apparatus. In particular, the present invention relates to a technique of improving the conversion efficiency of a power conversion apparatus in a light-load or no-load state.
2. Description of the Related Art
There is a switching power source apparatus employing a current-resonant power conversion apparatus. The current-resonant power conversion apparatus maintains a current resonance during a steady load state, to achieve zero-current switching (ZCS) and zero-voltage switching (ZVS) and realize low-noise, high-efficient power conversion.
The current-resonant power conversion apparatus always passes a resonant current without regard to aload condition. Namely, even during a standby state or the like in which there is light load or no load, the apparatus constantly passes a resonant current to cause a power loss to extremely deteriorate the power conversion efficiency of the apparatus. To improve the power conversion efficiency during a light-load or no-load condition, Japanese Unexamined Patent Application publication No. 2005-143255 (herein after referred to as Document D3) discloses a power conversion apparatus that performs an intermittent power conversion operation by temporarily stopping a power conversion operation (switching operation) during a light-load condition or no-load condition. The resonant-type power conversion apparatus that carries out such an intermittent power conversion operation is hereinafter referred to as “resonant power conversion apparatus with intermittent oscillation mode.
FIG. 1 is a block diagram showing a lamp lighting apparatus employing a resonant power conversion apparatus with intermittent oscillation mode according to a related art. The lamp lighting apparatus includes a DC power source Vdc and a series circuit connected between ends of the DC power source Vdc, the series circuit consisting of a high-side switching element Qh and a low-side switching element Ql. The switching elements Qh and Ql are, for example, MOSFETs. The DC power source Vdc consists of a rectifying-smoothing circuit that includes a diode bridge rectifier for rectifying commercial AC power and a smoothing capacitor for smoothing the output of the diode bridge rectifier (not shown in FIG. 1).
Between the source and drain of the high-side switching element Qh, a high-side clamping diode Dh is connected. The diode Dh may be replaced with a parasitic diode of the high-side switching element Qh. Between the source and drain of the low-side switching element Ql, a low-side clamping diode Dl is connected. The diode Dl may be replaced with a parasitic diode of the low-side switching element Ql.
Between the source and drain of the low-side switching element Ql, a voltage resonant capacitor Crv is connected. The capacitor Crv may be replaced with a parasitic capacitance of the high- and low-side switching elements Qh and Ql.
Between the source and drain of the low-side switching element Ql, an LC series resonant circuit (hereinafter referred to simply as “resonant circuit”) is connected. The resonant circuit includes a resonant capacitor Cri on the primary side of a transformer T and a primary winding N1 of the transformer T. As is well known, the transformer T includes an exciting inductance Lp and a leakage inductance Lr (not shown).
The transformer T has a secondary winding N2 whose ends are connected to an output circuit. The output circuit includes a resonant capacitor Cs on the secondary side of the transformer T. Ends of the resonant capacitor Cs are connected to a current detecting resistor Rref and a load L. The load L is a lamp. Between the load L and the resistor Rref, an error amplifier 11 is connected. The error amplifier 11 receives a voltage from a connection point of the resistor Rref and load L, compares the received voltage with a reference voltage, and outputs an error voltage that is sent to a controller 13.
To intermittently control power conversion, an intermittent signal generator 12 is arranged. The intermittent signal generator 12 generates an intermittent signal of instructing an intermittent operation and sends the signal to the controller 13. The intermittent signal is high when the load L (lamp) is ON and is low when the lamp is OFF. An OFF period or a low-level period of the intermittent signal is determined according to an external instruction or the state of the load L.
If the intermittent signal from the intermittent signal generator 12 indicates a steady load (i.e. not light condition nor no-load condition), the controller 13 generates a control signal whose pulse width (ON width) is proportional to the error voltage from the error amplifier 11, to alternately turn on/off the high- and low-side switching elements Qh and Ql. The control signal is sent to a driver 14.
If the intermittent signal from the intermittent signal generator 12 indicates light-load condition or no-load condition, the controller 13 generates a control signal to suspend the switching (on/off) operation of the high-side and low-side switching elements Qh and Ql without regard to the error voltage from the error amplifier 11 and sends the control signal to the driver 14.
According to the control signal from the controller 13, the driver 14 generates drive signals, i.e., gate signals Vg1 and Vg2 and supplies them to the gates of the high- and low-side switching elements Qh and Ql, respectively.
Operation of the lamp lighting apparatus according to the related art will be explained with reference to operational waveforms shown in FIG. 2. Operation under a steady load (operation up to to of FIG. 2) is well known to persons skilled in the art, and therefore, will not be explained.
The waveforms shown in FIG. 2 include a drain-source voltage Vds1 of the high-side switching element Qh, a drain current Id1 of the same, a drain-source voltage Vds2 of the low-side switching element Ql, a drain current Id2 of the same, the gate signal Vg1 to the gate of the high-side switching element Qh, the gate signal Vg2 to the gate of the low-side switching element Ql, a load current to the load L, and the intermittent signal from the intermittent signal generator 12.
According to the related art, the switching operation of the high-side and low-side switching elements Qh and Ql is suspended during a suspension period in which the intermittent signal is low (from t0 to t1). The suspension is achieved by lowering both the gate signal Vg1 to the high-side switching element Qh and the gate signal Vg2 to the low-side switching element Ql.
When the switching operation is suspended, the resonant circuit including the inductance Lp and resonant capacitor Cri holds energy. This energy results in a current passing through a path extending along Lp, Cri, Dh, Vdc, and Lp and is mostly consumed by regeneration at the DC power source Vdc to damp the resonance immediately.
With no energy being left in the resonant circuit, the switching operation is continuously suspended. At t1, the intermittent signal generator 12 raises the intermittent signal to trigger a start of oscillation. In response to this, the controller 13 sends a control signal to instruct the driver 14 to resume the switching operation of the high- and low-side switching elements Qh and Ql.
The lamp lighting apparatus of this related art controls a voltage supplied to the load L, and at the same time, intermittently conducts power conversion by intermittently suspending the switching operation of the high- and low-side switching elements Qh and Ql. The related art is simply configured to control an output, reduce an average power consumption, and improve a conversion efficiency.
Another related art is disclosed in Japanese Unexamined Patent Application Publication No. 8-66025 (hereinafter referred to as Document D1). This disclosure is a resonant-type switching power source apparatus employing a half bridge configuration that is compact, low-cost, and noise-resistive. The apparatus includes a DC power source and a series circuit connected to the DC power source, the series circuit including first and second transistors. In parallel with the second transistor, a primary winding of a transformer is connected. In series with the primary winding, a series resonant capacitor and an inductance are connected. The first transistor has no parallel auxiliary capacitor and only the second transistor is connected to an auxiliary capacitor in parallel.
Japanese Unexamined Patent Application Publication No. 11-164554 (hereinafter referred to as Document D2) discloses a current-resonant power source circuit. According to the disclosure, a range in which the operation frequencies of charge and discharge switching units are higher than a resonant frequency is extended toward a lower band side. The two switching units are turned on/off to resonate a primary winding of a switching transformer and a current resonant capacitor. In response to the resonance, a secondary winding of the switching transformer provides a high-frequency output. The current resonant capacitor is selected so that the capacitance thereof increases as the operation frequencies of the two switching units decrease.
The Japanese Unexamined Patent Application Publication No. 2005-143255 (Document D3) mentioned above also discloses a method of controlling a resonant power source apparatus. The method is capable of immediately stopping the resonance of a resonant circuit as soon as an active state is stopped. According to the method, the resonant circuit is arranged on the primary side of a transformer and a resonant current passing through the resonant circuit is controlled by driving a switching unit. The secondary side of the transformer is connected to an output circuit. During a driving state of the resonant power source apparatus, the switching unit is driven to keep the resonant current at a predetermined amplitude so that the output circuit may output a constant voltage. To stop the resonant power source apparatus, the switching unit is driven in a phase opposite to the driving state, so that the resonant current may have a zero amplitude.